1. Field of the Invention
The present invention relates to a method of producing an inflation film, an apparatus therefor, and an inflation film obtained thereby. More specifically, the invention relates to a method of producing an inflation film of which the mechanical properties such as tensile strength, impact strength, etc. and the thickness can be adjusted over wide ranges, to an apparatus therefor, and to an inflation film having excellent properties obtained thereby.
2. Description of the Prior Art
An ultra-high-molecular-weight polyolefin features superior impact strength, abrasion resistance, resistance against chemicals and tensile strength to those of general-purpose polyolefins, and is finding spreading applications as an engineering resin. However, the ultra-high-molecular-weight polyolefin exhibits a very higher melt-viscosity and poorer fluidity than those of the general-purpose polyolefins. Therefore, the ultra-high-molecular-weight polyolefin alone cannot be molded by using a molding machine for general-purpose polyolefins.
Therefore, the present applicant has previously proposed a method of producing a biaxially drawn film by extrusion-molding the ultra-high-molecular-weight polyolefin to which is mixed large amounts of a plasticizer (Japanese Patent Publication No. 16330 /1992). When this method is employed, however, a plasticizer must be extracted from the obtained polyolefin film depending upon the applications.
The present applicant has further proposed a method of producing an inflation film by using a tube die having a mandrel that rotates accompanying the rotation of a screw of an extruder without using a plasticizer (Japanese Patent Publication No. 55433 /1994).
According to this method, however, the tubular film that is extruded is turned sideways, permitting the upper portion to become thin and lower portion to become thick, causing a difference in the thickness between the upper portion and the lower portion. In order to increase the rate of production, furthermore, the rotating speed of the mandrel must be increased accompanying an increase in the rotating speed of the screw giving rise to the occurrence of a problem in that the resin is deteriorated due to friction. In order to erase flight marks of the resin, furthermore, it is required to employ a mandrel of an increased length. There is also a problem that the apparatus becomes large-sized and its installation space becomes wide.
In order to provide optimum inflation films depending upon the applications, furthermore, it becomes necessary to adjust the mechanical properties such as tensile strength and impact strength as well as the thickness of the obtained inflation films over wide ranges.
In view of the above-mentioned technical assignments, the present inventors have conducted study to solve problems, and have found the fact that the mechanical properties such as tensile strength and impact strength, drawing ratio in machine direction and thickness can be adjusted over wide ranges, and an inflation film can be formed permitting the molecular weight to drop less than those of the conventional film-producing methods, by melting a thermoplastic resin in a extruder, by using a screw die having an L/D of at least 5 and having a mandrel which rotates independently of the screw of the extruder, the mandrel rotating at a speed as low as possible, and have confirmed that the inflation film exhibits excellent properties of an ultra-high-molecular-weight polyolefin, and have thus arrived at the present invention.